1. Field of the Invention
The present invention relates to a vacuum arc deposition device for forming films by utilizing a vacuum arc phenomenon.
2. Description of the Prior Art
Among various methods for forming films in vacuum, a vacuum arc vapor deposition method has industrially been used as a method capable of efficiently forming a hard film such as TiN. The vacuum arc vapor deposition is executed according to such a method as to generate a so-called vacuum cathode arc discharge between a cathode and an anode disposed in a vacuum vessel and to evaporate the cathodic material from an arc spot formed on the surface of the solid cathode, thus depositing the resultant vapor on a substrate disposed in the vacuum vessel to form a film.
As a device for realizing the vacuum arc vapor deposition, there have been known such devices as disclosed in Japanese Patent Publications Nos. Sho 58-3033 and sho 52-14690, which have been variously improved after that. As an evaporation source for a vacuum arc vapor deposition as shown in FIG. 7 is well-known to those skilled in the art.
In FIG. 7, a vacuum arc evaporation source 1 disposed in a vacuum vessel 2 has a cathode 3 and an anode 4. An arc discharge power source 5 is connected between the cathode 3 and the anode 4. A target 6 comprising a material to be evaporated is disposed to the cathode 3. When a vacuum arc discharge is generated under a voltage of several tens of volts and with a current from several tens to several hundreds of amperes, by an ignition mechanism (not illustrated), an arc spot is generated on the surface of the target 6 and vapor is emitted therefrom. The vapor is made to deposit on a substrate 7, to thereby form a film 8. An arc stabilizing mechanism 10 is disposed around the target 6 with an aim for maintaining the surface of the target 6 as a predetermined evaporation surface 9 (disclosed, for example, in Japanese Patent Laid-Open No. Sho 59-208070).
In the drawing, are also shown a vacuum pump 11 and a bias power source 12.
The vacuum arc vapor deposition may provide a high vapor ionization ratio so as to obtain a dense film of high quality and also provides a high film formation rate. Further, the area and shape of the evaporation surface 9 of the target 6 is not constrained by the evaporation process, and many target shapes and sizes may be used. For example, the evaporation surface 9 can be constituted as a circle of 100 mm diameter or as a rectangular shape with 1 m length in the longitudinal direction. The evaporation source 1 having a rectangular evaporation surface 9 is useful for forming a deposition film on a large-sized member or on a sheet-like product of large width, such as, a steel sheet.
However, there has been the following problem in a vacuum arc evaporation source when the evaporation source has a large area. As the evaporation surface is enlarged, the arc average current density at the surface of the evaporation surface is reduced in a reverse proportion with the area so that the vapor deposition rate in front of the evaporation surface is correspondingly reduced. As one of countermeasures therefor, a method for increasing the arc discharge current in accordance with the increase in the evaporation surface may be considered. Indeed, this method can achieve an advantageous effect within a certain extent of an area ratio, but it brings about another problem as the area is further increased.
Namely, an arc current of about 100A is typically required for an evaporation source having an evaporation area of about 100 mm diameter, which is the surface area of targets which are most commonly used. From the simple proportional rule, the discharge current of 300A would be required for an evaporation source having three times of the above area. Further, for an evaporation source having ten times of the above area, the discharge current of 1000A would be required. Although arc discharge at 300A can be coped with using the existent technique, the arc discharge at 1000A or greater causes the following practical problems which are particularly important in an evaporation source for continuous operation.
1. The arc discharge current becomes huge.
2. The diameter of a cable transmitting the arc power is increased along with an increase in current to result in inconvenience for handling. Since the cross section of a cable conductor required for the power transmission is increased in proportion with the square of electric current, wiring has to be done not by usual cables but by bus bars, particularly, in a region in which the current exceeds 1000A, which increases the difficulty in an assembling work.
3. As the current flowing in the cable increases, the intensity of a magnetic field caused thereby is also increased, which gives an effect on the movement of an arc spot to result in undesired effect such as localization of discharge.
4. Even if an evaporation surface of a large area is provided, the arc spot tends to localize to a portion of an evaporation surface to possibly cause uneven evaporation.
5. As the discharge current becomes greater, a current concentrated portion is formed not only on the surface of the cathode but also on the surface of the anode (anode spot) to possibly damage that portion.
Accordingly, even if an evaporation source of a large evaporation area is used, since the arc discharge current can not be increased in proportion with the evaporation area by the reason described above, there has been a problem that the film formation rate in average is reduced. As a result, in an application use requiring a high film formation rate, a necessary film formation rate can not be attained by an evaporation source of a large area. Therefore, large-scaled evaporation source can be constituted.